Homocysteine remethylation during nitrous oxide exposure of cells cultured in media containing various concentrations of folates.

Nitrous oxide irreversibly inactivates cob(I)alamin, which serves as a cofactor of the enzyme methionine synthase catalyzing the remethylation of homocysteine to methionine. In patients exposed to nitrous oxide, increase in plasma homocysteine is a responsive indicator of cob(I)alamin inactivation. In the present work, we measured the inactivation of methionine synthase and the concurrent homocysteine export rate of two murine and four human cell lines during nitrous oxide exposure. When cultured in a standard medium with high content (2.3 microM) of folic acid, the methionine synthase of all cell types was inactivated at an initial rate of 0.05 to 0.14 h-1. The inactivation curves leveled off, and a residual activity of 15 to 45% was observed after 48 h of nitrous oxide exposure. The rate and extent of the nitrous oxide-induced inactivation were markedly reduced when the cells were transferred and cultured (greater than 10 days) in a medium containing low concentration (10 nM) of 5-methyltetrahydrofolate. The methionine synthase inactivation increased in a dose-dependent manner when the 5-methyltetrahydrofolate content of the medium was increased from 3 nM to 2.3 microM. The inactivation of methionine synthase was associated with a marked enhancement of homocysteine export rate of murine fibroblasts and a moderate increase in export from two human glioma cell lines. In contrast, in three leukemic cell lines (murine T-lymphoma R 1.1 cells, human promyelocytic leukemia HL-60 cells and human acute myelogenous leukemia KG-1a cells), the homocysteine export rates were not increased during nitrous oxide exposure. In the responsive murine fibroblasts and the glioma cells, the homocysteine export rate varied inversely to the changes in methionine synthase activity induced by nitrous oxide exposure at different concentrations of folate in the medium. The enhancement of homocysteine export rate of some cell types during nitrous oxide exposure probably reflects inhibition of homocysteine remethylation in intact cells, and highlights the utility of extracellular homocysteine as an indicator of metabolic flux through the methionine synthase pathway. No enhancement of homocysteine export despite inactivation of methionine synthase in three leukemic cell lines questions the functional state of the enzyme in these cells.